Wireless communication systems are widely deployed to provide various types of communication content, such as voice, data, and so on. Typical wireless communication systems are multiple-access systems capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power, etc.). One class of such multiple-access systems is generally referred to as wireless location area networks (WLANs), such as “Wi-Fi,” and includes different members of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless protocol family. Generally, a Wi-Fi communication system can simultaneously support communication for multiple wireless devices, such as wireless stations (STAs). Each STA communicates with one or more access points (APs) via transmissions on the downlink and the uplink. The downlink (DL) refers to the communication link from the APs to the STAs, and the uplink (UL) refers to the communication link from the STAs to the APs.
Modern navigation systems have typically used satellite-based global positioning system (GPS) for position determination. However, the recent proliferation of WLAN (e.g., Wi-Fi) access points has made it possible for navigation systems to use these access points for position determination, especially in urban areas where there is usually large concentration of WLAN access points. WLAN navigation systems can be advantageous over GPS navigation systems because of limitations of GPS signal coverage. For example, while GPS signals may not be readily available inside a shopping mall, wireless signals generated by WLAN access points inside the shopping mall would be more readily detectable by a STA.
More specifically, for WLAN navigation systems, the locations of the WLAN access points are used as reference points from which well-known trilateration techniques can determine the location (e.g., absolute location and/or relative location) of a wireless device (e.g., a Wi-Fi-enabled cell phone, laptop, or tablet computer). The wireless device can use the round trip time (RTT) of signals transmitted to and from the access points to calculate the distances between the wireless device and the access points. Once these distances are calculated, the location of the wireless device can be estimated using trilateration techniques.
One procedure for determining RTT captures the amount of time between the transmission of a unicast packet, such as a data packet or request to send (RTS), by STA to an AP and the reception of the appropriate response packet, which may be an acknowledgement (ACK) or clear to send (CTS), as measured by the STA. RTT is typically measured in nanoseconds, but can be done in picoseconds.
Some chip designs may allow recording of the time of departure (TOD), from the STA and also time of arrival (TOA) at the STA, using timestamps. The timestamps permit measurement of RTT. This method may be referred to as an RTS/CTS or QOS Null/Ack-based RTT procedure (also referred to herein as a non-fine timing measurement (FTM) procedure).
However, in measuring RTT there is a variable amount of turnaround time delay involved at the AP, or receiving node, that needs to be accounted for before the RTT may be used for ranging calculations. These ranging calculations using RTT are made by extracting the time of flight between the STA and the AP, which requires knowledge of the turn-around calibration factor (TCF). TCF may be implementation specific and may depend on the short interframe space (SIFS), time of arrival uncertainty, and other delays at the AP. The TCF varies depending on the chipset used by the AP.
Another method for determining the range between nodes is often referred to as the Fine Timing Measurement (FTM) protocol. Based on FTM, a STA exchanges FTM messages with the AP and then receives timing information (e.g., timestamps corresponding to departure time of the FTM message and the arrival time of the Ack to the FTM message at the AP) from the AP. The STA then computes its range to the AP based on the timing information.
While FTM procedure may offer increased accuracy and/or reliability for position determinations, some legacy STAs are not configured to use the FTM procedure. Thus, when a legacy STA uses an FTM-capable AP for positioning, the legacy STA will still have to use the non-FTM procedure (e.g., the RTS/CTS or QOS Null/Ack-based RTT procedure). However, as mentioned above, the RTS/CTS-based RTT procedure requires knowledge of the APs TCF (i.e., the APs turn-around calibration factor).